But they aren't the same. Kernels are different, along with binaries. Mac OSX has the 4.4- now defunct- BSD/Mach kernel. DOS was/is a system. I used it with old ataris.
A talk on linux would be a long one. A talk on UNIX would be longer quite a number of systems to cover.
DOS has a bit iof historical significance.
Hmm. Any less than a day covered on any of these subjects would be too short.